Multi-Party Quorum Authorization for High-Risk Agent Actions (EP-QUORUM)

Introduction The base EP authorization receipt closes the gap between "is this actor authorized in general?" and "should this exact action happen, and which accountable human said yes?" by binding one named approver's device-held signature to one exact action (see , Section 1). For the highest-consequence actions, one approver is not the right control. The discipline that governs nuclear release, large-value treasury movement, and production-credential change is the two-person rule: no single human -- however well-authenticated, however senior -- can unilaterally cause the action. Two or more distinct, accountable humans must each independently authorize, and the action proceeds only when all of them have. As autonomous agents acquire credentials sufficient for irreversible operations, the two-person rule is exactly the control that lets an organization grant an agent real authority without creating a single point of failure: a compromised, misaligned, or prompt-injected agent cannot act alone, and neither can a single compromised or coerced approver. EP-QUORUM specifies how to express that control as a cryptographic predicate over EP signoffs and how to enforce it both at the moment each approver signs and at the moment the action would execute. The base draft already contemplates multi-approver policies (, Section 7): each approver signs an individual Authorization Context sharing the same action hash, and commitment occurs only when k valid, distinct signoffs exist before expiry. This document makes that sketch normative and testable. It adds: ordered approval trails (); an explicit role roster and admission semantics (); a bounded approval window with a monotonic-time constraint for ordered mode (); an incremental server-side admission rule that keeps a non-conforming signer out of the trail in the first place (); the consolidated fail-closed quorum predicate (); and an adversarial conformance suite ().

Design Goals EP-QUORUM inherits design goals G1-G7 of the base draft and adds:

Q1 -- Additivity. A quorum is a set of unmodified EP signoffs over the same action hash. No new signature type, no new signing ceremony, and no change to the receipt verifier are introduced. A single-approver policy is the one-member quorum.
Q2 -- Fail-closed. Authorization is denied unless every element of the quorum predicate holds. Absence of evidence, an unparseable member, a malformed policy, or any single failed check yields "not satisfied," never "satisfied."
Q3 -- Distinctness (separation of duties at the human level). A quorum of size k requires k pairwise-distinct human approvers, each distinct from the initiator. One human MUST NOT fill two slots.
Q4 -- Incremental enforcement. The protocol enforces conformance as each approver signs, not only at consume time, so that a wrong-action, wrong-role, duplicate, out-of-order, stale, or invalid signature never becomes part of the trail.
Q5 -- Offline-verifiable quorum. The satisfied/not-satisfied judgment is computable from the receipt's members and the policy alone, under the same offline verification model as the base draft (, Section 6.3).

Terminology In addition to the terminology of : The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHOULD", "MAY" are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals.

Quorum.: The set of distinct approver signoffs required to authorize one action under a Quorum Policy.
Quorum Policy.: A named, versioned rule set declaring the approval mode, the required count, the roster of eligible (role, approver) slots, the distinct-humans rule, and the approval window. Carried in the policy that governs the action; see .
Member.: One element of a candidate quorum: a (role, approver public key, signoff) triple, where the signoff is an unmodified EP signoff (, Section 5.3) over the Authorization Context the approver signed.
Trail.: The ordered sequence of members admitted so far for one action -- the partial quorum under construction.
Quorum Gate.: The fail-closed predicate () that decides whether a trail is a satisfied quorum. The Verifying Executor (, Section 9) MUST consult it before performing the action.

The Quorum Policy A Quorum Policy is a JSON object: Members:

Field	Required	Type	Meaning
`mode`	REQUIRED	string (enum)	`threshold` or `ordered` ().
`required`	REQUIRED	integer >= 1	The quorum size k.
`approvers`	REQUIRED	array	The roster of eligible {role, approver} slots. MUST be non-empty.
`distinct_humans`	OPTIONAL (default true)	boolean	When true, no approver identifier may fill more than one slot. Implementations MUST treat a missing value as true.
`window_sec`	OPTIONAL (default 900)	integer > 0	Maximum span, in seconds, between the first and any later admitted signature.

Rules:

An approver slot identifies an eligible approver in the Approver Directory (, Section 5.2). The role is the organizational role under which that approver is admitted to this quorum; it is the unit of role eligibility (, check 4).
A member is admitted only if its (role, approver) pair is present in the roster. A correct signature by a real, enrolled approver who is not on the roster for this action MUST be rejected (wrong_role).
required MUST NOT exceed the number of distinct human approvers the roster can supply under distinct_humans. A policy that cannot be satisfied is a misconfiguration; verifiers treat an unsatisfiable trail as not satisfied, as always.
The Quorum Policy is part of the action's governing policy and is therefore committed by the policy_hash of every member's Authorization Context (, Section 4). A signature collected under one Quorum Policy version MUST NOT satisfy a requirement evaluated under another.

Approval Modes Threshold mode ("threshold"). Any required distinct approvers drawn from the roster, in any order, satisfy the quorum. This is the classic M-of-N rule. Ordered mode ("ordered"). Approvals MUST occur in the roster's listed order: the i-th admitted member MUST match approvers[i-1] in both role and approver. In addition, signature times MUST be strictly increasing -- each member's issued_at MUST be later than the previous admitted member's issued_at (non_increasing_time). Ordered mode expresses escalation chains in which a later authority signs after, and in knowledge of, an earlier one (e.g., Program Officer, then Authorizing Official, then Inspector General). In both modes, every admitted member's issued_at MUST fall within window_sec of the first admitted member's issued_at (window_exceeded). The window bounds the lifetime of a partial quorum so that a long-dormant partial trail cannot be completed much later by an attacker who has since compromised a remaining approver.

The Quorum Gate (fail-closed predicate) A trail is a satisfied quorum for an action with hash H under policy P if and only if ALL of the following hold. A verifier MUST return "satisfied" only when every check passes, and MUST return "not satisfied" on the first failure, on a malformed policy or member, or on any unrecognized condition (Q2):

Well-formed policy. P has a recognized mode, an integer required >= 1, and a non-empty approvers roster. Otherwise: not satisfied.
All signatures valid. For every member, the EP signoff verifies under , Section 5.3 / 6.3 -- the WebAuthn assertion (Class A) verifies against the member's approver_public_key, with the assertion challenge equal to the member's context hash and user verification asserted. One invalid signature (one_bad_signature) fails the whole quorum.
Action binding. Every member's Authorization Context carries action_hash == H (action_mismatch). A member bound to any other action does not count.
Role admission. Every member's (role, approver) pair is present in the roster (wrong_role).
Distinct humans. When distinct_humans is true (the default), approvers are pairwise distinct and (per the base draft's SelfApprovalImpossible) distinct from the initiator (duplicate_human).
Threshold. At least required admitted members exist (under_threshold).
Order (ordered mode only). The i-th admitted member matches approvers[i-1]; signature times are strictly increasing (out_of_order, non_increasing_time).
Window. Every admitted member's issued_at is within window_sec of the first member's issued_at (window_exceeded).

The predicate is the same whether computed by the orchestrating operator before consumption or by an independent Verifying Executor or auditor offline (Q5): it is a pure function of (P, H, members). Because each member is an unmodified EP signoff, check 2 is exactly the base draft's verifier invoked per member; EP-QUORUM adds the set-level checks 1, 3-8 on top.

Incremental Admission (canAccept) To keep a non-conforming signer out of the trail rather than discovering it only at consume time (Q4), an orchestrator MUST evaluate an incremental admission rule before recording each new signoff. Given the policy P, the action hash H, the already-admitted trail, and one incoming candidate member, the rule ADMITS the candidate only if all of the following hold, and otherwise REJECTS it with the named reason:

P is well-formed and its roster is non-empty (else no_policy / no_eligible_approvers).
The candidate's context carries action_hash == H (else action_mismatch).
The candidate's (role, approver) is on the roster (else ineligible_role).
When distinct_humans is true, no already-admitted member shares the candidate's approver (else duplicate_human).
In ordered mode, the candidate matches the next unfilled roster slot (approvers[len(trail)]) in both role and approver (else out_of_order).
If the trail is non-empty, the candidate's issued_at is within window_sec of the first member's issued_at (else window_exceeded); and in ordered mode it is strictly greater than the last admitted member's issued_at (else non_increasing_time).
The candidate's signature verifies (else invalid_signature).

A rejected candidate MUST NOT be written into the trail. Incremental admission is an enforcement convenience and an early-rejection UX; it is not a substitute for the Quorum Gate. A conforming Verifying Executor MUST re-evaluate the full Quorum Gate () over the assembled trail before performing the action, regardless of incremental admission, because the executor does not trust the orchestrator to have applied admission honestly (this mirrors the base draft's execution-side enforcement, , Section 9).

Member Representation in the Receipt A quorum receipt is an ordinary EP Trust Receipt (, Section 6.2) whose contexts and signoffs arrays carry one entry per admitted member, plus the Quorum Policy (by reference via policy_hash, and optionally inline for convenience). For the offline quorum computation, each member is the triple: ", "signoff": { "@type": "ep.signoff", "context": { "context_type": "ep.signoff.v1", "action_hash": "...", "approver": "...", "issued_at": "...", "...": "..." }, "webauthn": { "authenticator_data": "...", "client_data_json": "...", "signature": "..." } } } ]]> The context and webauthn members are exactly as defined by the base draft; EP-QUORUM does not alter their canonicalization, hashing, or signature verification. The role and approver_public_key are the join keys against the Quorum Policy roster and the Approver Directory.

Conformance An implementation conforms to EP-QUORUM if, for the published adversarial conformance vectors, it returns the expected satisfied/not-satisfied verdict for every vector and rejects every non-conforming candidate at incremental admission with the expected reason. The reference suite (EP-QUORUM-v1) comprises the following vectors, each carrying real Class-A WebAuthn assertions:

Vector	Expect	Exercises
`accept_ordered_3of3`	satisfied	Ordered PO, AO, IG; distinct; increasing time; all action-bound
`accept_threshold_2of3`	satisfied	Any 2 distinct approvers from a 3-slot roster
`reject_under_threshold`	not satisfied	Fewer than `required` valid members
`reject_duplicate_human`	not satisfied	One human filling two slots
`reject_out_of_order`	not satisfied	Ordered mode, members out of roster order
`reject_action_mismatch`	not satisfied	A member bound to a different action hash
`reject_expired_window`	not satisfied	A member outside `window_sec`
`reject_one_bad_signature`	not satisfied	One invalid member signature
`reject_wrong_role`	not satisfied	A correct signature by an off-roster approver

The reference suite is maintained such that three independent implementations (JavaScript, Python, Go) MUST agree on every vector; divergence is a conformance defect in at least one implementation. The "accept" vectors guard against a verifier that is too strict (denying valid quorums); the "reject" vectors guard against a verifier that is too lenient (the security-critical direction).

Security Considerations EP-QUORUM inherits all Security Considerations of and adds the following. Several restate, honestly, what a quorum does not buy.

What multi-party authorization does and does not prevent A satisfied quorum proves that k pairwise-distinct enrolled approvers each produced a valid, action-bound, in-window, in-order (where required) signature with their own device-held keys, and that the orchestrator could not have forged any of them (, Section 11.1). It raises the cost of unilateral action: a single compromised agent, a single stolen key, a single coerced or malicious approver is insufficient. It does not defeat collusion among the required number of distinct humans, nor one human who controls multiple enrolled identities (an enrollment control -- base draft Section 5.2), nor simultaneous coercion of a full quorum. As in the base draft, EP-QUORUM makes such events attributable -- named, signed, and evidenced for every member -- which is a deterrent and an audit primitive, not an impossibility proof. Implementations MUST NOT claim a quorum is collusion-proof.

Fail-closed is the only safe default The dangerous error in a multi-party gate is to treat ambiguity as approval. EP-QUORUM is specified so that a malformed policy, a missing or unparseable member, a partial trail, or any single failed check yields "not satisfied." A verifier MUST NOT default to satisfied on any unrecognized condition. The "reject" conformance vectors exist to catch a regression in this direction.

Partial trails confer no authority A trail short of the threshold, or one in which incremental admission has accepted some but not all required slots, authorizes nothing. A Verifying Executor presented with a partial trail MUST refuse, exactly as it refuses a missing single signoff. This is the multi-party form of the base draft's NoBypassWrite invariant.

Window and replay The approval window (window_sec) bounds how long a partial quorum remains completable, limiting the value to an attacker of compromising a remaining approver after some approvals already exist. Each member's signoff retains the base draft's one-time-consumption nonce (G3); the window is an additional, quorum-level constraint, not a substitute for per-signoff replay protection. The monotonic-time rule in ordered mode additionally prevents back-dating a later authority's signature to appear to precede an earlier one.

Divide-and-misinform across members Because each approver signs their own Authorization Context, a malicious orchestrator can attempt to show different approvers different renderings or different initiator attestations (, Section 11.9) while each individual signature remains valid. EP-QUORUM does not change the base draft's cross-context consistency requirement; verifiers SHOULD surface per-member context differences, and high-value ordered policies SHOULD render the prior approvers' decisions to each subsequent approver so that the trail is a chain of informed approvals rather than parallel ones. The presentation-attack mitigations of base draft Section 11.3 apply per member.

Approver fatigue, at quorum scale Requiring more humans does not help if each rubber-stamps; it can hurt, by diffusing responsibility across a group in which no member feels decisive (base draft Section 11.8). Quorum policies MUST be scoped to genuinely high-consequence, low-frequency actions, and deployments SHOULD monitor per-role time-to-sign and deny rates rather than assume that more signers means more scrutiny.

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